Semiconductor memory devices include volatile semiconductor memory devices and non-volatile semiconductor memory devices. A volatile semiconductor memory device typically has fast read and write speeds, but drawbacks in that stored content is lost when external power supply is discontinued. In contrast, a non-volatile semiconductor memory device, which may have read and write speeds that are relatively slow, the stored content may be kept even when the external power supply is discontinued. Thus, to store data to be stored regardless of the continuation of power supply, the non-volatile semiconductor memory device is used.
Nonvolatile semiconductor memory devices include phase-change random access memory (PRAM), mask read-only memory (MROM), programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), and electrically erasable programmable read-only memory (EEPROM), etc. With respect to MROM, PROM, and EPROM among these nonvolatile semiconductor memory devices, it is not easy for normal users to renew stored information since erase and write operations cannot be freely performed. On the other hand. EEPROM enables electrical erase and write operations and is thus increasingly used for system programming needing continuous update or used as auxiliary memory devices. Particularly, flash EEPROM (hereinafter, referred to as a flash memory device) has a higher degree of integration than existing EEPROM and is thus very advantageous when it is used as a large-capacity auxiliary memory device. Accordingly, the flash memory device is widely used in small electronic devices. For example, flash memory devices may be used in digital cameras, digital camcorders, digital music players.
A flash memory device determines whether electrons are injected into a floating gate using a Fowler-Nordheim (F-N) tunneling mechanism and stores data at a first logic level or a second logic level according to a result of the determination. Recently, studies on multi-level memory for storing multi-bit data in a single memory cell have been actively carried on in order to increase the density of integration for memory. A memory cell in the multi-level memory is capable of storing multiple bits. Such memory cell storing multiple bits is referred to as a multi-level cell (MLC) while a memory cell storing a single bit is referred to as a single-level cell (SLC). Since the multi-level cell stores at least two bits, it has at least three data storage states (or cell states or states) and at least three threshold voltage distributions respectively corresponding to the states.
The threshold voltage distribution of a multi-level cell can change over time due to coupling with an adjacent cell, charge loss, and so on. The variation of a cell distribution is related with a cell state. For instance, a cell state having a high threshold voltage induces more charge loss than a cell state having a low threshold voltage and a coupling effect increases when an adjacent cell having a state corresponding to a low threshold voltage distribution is programmed to a state corresponding to a high threshold voltage distribution.